1. Field of the Invention
The present invention relates to a balun used in telecommunications for feeding electric power to a balanced line from an unbalanced circuit, a power feeder consisting of a microstrip line.
2. Description of Related Art
When using a high frequency band in telecommunications, a microstrip line is often employed as a transmission line for conveying a signal. On the other hand, there is a balanced circuit such as an antenna for transmitting or receiving the signal by radio via space. A balun is used to connect such a balanced circuit with the microstrip line.
FIG. 18 shows a conventional balun. In FIG. 18, the reference numeral 1 designates a balanced line; and 2 designates a microstrip center conductor of a microstrip line which constitutes an unbalanced circuit together with a ground plate not shown to transmit a signal. The reference numeral 3 designates a connecting point of the balanced line 1 and microstrip center conductor 2; and 4 designates an additional line for short-circuiting the balanced line 1 at a position one quarter wavelength apart from the connecting point 3. The reference numeral 5 designates a grounding terminal for grounding the balanced line 1 at a position half wavelength apart from the connecting point 3. Because the balanced line 1 is terminated by the short-circuit at one quarter wavelength apart and by the ground at half wavelength apart from the connecting point 3 on the additional line 4 side, a current (unbalanced current) fed from the microstrip center conductor 2 is transformed to a current (balanced current) flowing through the balanced line 1 in opposite directions. With such a structure, the conventional balun must comprise on the balanced line additional circuits such as the quarter-wave transmission line, half-wave transmission line and additional transmission line, and this presents a problem of increasing the dimension of the structure. Furthermore, because the additional circuits are determined in accordance with the wavelength of the signal to be transmitted, and cannot transform signals of other frequencies from an unbalanced to balanced current, a problem arises of restricting the application of the balun fabricated to a particular frequency.
The present invention is implemented to solve the problems. Therefore, an object of the present invention is to provide a compact, rather frequency independent balun.